Hi all,
Some version of Arduino, lets say UNO has a LED series with 1k resistor in-built SMD LED for its 13th output pin. The 'Blink' sample program of Arduino IDE uses the 13th pin to blink its built-in LED as default.

Question is: The 13th pin of the UNO is not completely dedicated to the LED. So we use the 13th pin in many purpose. But as I said the 13th pin has built-in LED connected permanently. Won't it affect to another signal when we use the 13th pin for different signal output?

Some version UNO do not have built-in LED in board for 13th pin.

 

No doubt the controller has way more output current capability than is required to drive the LED (from memory 50 ma max) so yes, you can use this as an output with very little limitation. If you use that pin as an input whatever signal is driving that input will also have to drive the LED.

 

No. just like if you were to add an LED in parallel with a j-random circuit as an indicator,the permanent LED will not affect something that is connect in parallel with it. The entire circuit may draw a tiny bit more current, but that is all.

 

Each pin on the Arduino can source 40mA with an total upper limit of 200mA